1. Field of the Invention
This present invention relates to hydroxy-functional prepolymers containing at least two urea groups, which may be converted to thermally stable hydantoin groups at elevated temperatures, and their use in combination with optionally blocked polyisocyanates and/or aminoplast resins for the production of polyurethanes and polyamides, respectively, especially as binders in coating compositions.
2. Description of the Prior Art
Coating compositions containing, as binders, optionally blocked polyisocyanates and/or aminoplast resins in combination with polyether, polyester or polycarbonate polyols are known. These compositions may also be used for the production of elastomers, potting compounds, composite matrices and in other related applications. One of the deficiencies of using these known polyols is that they do not possess sufficient thermal, chemical and hydrolytic stability.
Accordingly, it is an object of the present invention to provide improved polyol co-reactants for optionally blocked polyisocyanates or aminoplast resins which may be used to prepare products with improved thermal, chemical and hydrolytic stability.
This object may be achieved with the hydroxy-functional prepolymers according to the present invention, which contain urea groups that may be converted to thermally stable hydantoin groups at elevated temperatures.
The reaction of polyaspartates with polyisocyanates to form coatings is disclosed in U.S. Pat. No. 5,126,170. The polyisocyanates are blended with polyaspartates and then reacted after the mixture has been applied to a suitable substrate to form a urea group-containing coating. The coating is cured at low temperatures so that conversion of the urea groups to hydantoin groups does not take place.
In German Offenlegungsschrift 2,158,945 polyisocyanates are reacted with .beta.-aminocarboxylic acid derivatives (which broadly encompass aspartates, see Example 7) to form open chain urea derivatives, which may subsequently be heated to form 6-membered 2,4-dioxohexahydro-pyrimidine derivatives.
U.S. Pat. No. 3,639,418 is directed to the reaction of bis-aspartates with monoisocyanates to form a urea intermediate which is then converted into the corresponding hydantoin by heating at elevated temperatures.
U.S. Pat. No. 3,549,599 is directed to carboxylic acid ester substituted polyhydantoins prepared by reacting stoichiometric amounts of polyaspartates with polyisocyanates and subsequently converting the urea groups to hydantoin groups. Unless chain terminating monoaspartates are used during their production, the resulting products are high molecular polymers, which may be crosslinked through the ester group remaining after hydantoin formation by transesterification or aminolysis reactions. In addition, this reference is primarily directed to the use of aromatic polyisocyanates to prepare the polyhydantoins. It can be shown that such polyhydantoins are inferior to the corresponding polyhydantoins prepared from (cyclo)aliphatic polyisocyanates with regard to viscosity and color of the polyhydantoins and the flexibility, color and weathering of the polyurethanes and polyamides prepared therefrom.
None of the preceding references suggests the preparation of the hydroxy-functional prepolymers according to the present invention or their use as co-reactants for optionally blocked polyisocyanates and/or aminoplast resins.